Geochemical characteristics of Callovian—Oxfordian carbonates in Samandepe gas field, Amu Darya Basin, Turkmenistan

Abstract

The Callovian-Oxfordian carbonate reservoirs are the most important hydrocarbon reservoir in the Samandepe gas field, Amu Darya basin, Turkmenistan. Based on the analysis of Fe, Mn and Sr trace elements, and carbon, oxygen and strontium isotopes, the genesis and evolutionary characteristics of the carbonate reservoirs were studied, and the conclusions were follows: 1) Sustained transgressive-regressive cycles played an important role during Callovian-Oxfordian. The reservoir of reef-bank facies was well de veloped in the period of transgression, while the regional dense cap rocks developed in the period of regression; 2) The 87Sr/86Sr ratio measured from rudist shells yields an age of 157.2 Ma according to the global strontium isotope curve; 3) As diagenetic intensity increased, δ13C changed little, and δ18O showed strong negative deviation but was still limited to the range of Late Jurassic seawater. High Fe and Sr contents, and low Mn content, and the evolutionary trend of δ13C and δ18O all indicate that diagenesis occurred in a relatively confined environment, where the fluids were relatively reducing and contained hot brine. The stage of diagenesis reached is mesodiagenesis, which is very favorable for preservation of primary pores in carbonates; 4) Strong dissolution of reef limestones, burial dolomitization and hydrothermal calcite precipitation were all the results of the interaction between the 87Sr-rich diagenetic fluid and rocks; 5) The analysis results of isotopes, fluid inclusions and vitrinite reflectance show that the diagenetic fluid was compaction-released water that originated from the deep, coal-bearing clastic strata.